Iron hydroxide is particularly difficult to remove from water. Iron hydroxide is a gelatinous sticky substance that occupies much larger volumes than its concentration by weight would indicate. Consequently, it blinds faces of surface type filters and plugs depth filters. Diatomaceous earth is often used as a filter aid to remove solids and iron solids in particular from domestic water supplies and from oil field fluids, but with less than desirable results. Although effective to remove most solids, diatomaceous earth is not particularly effective at removing iron hydroxide due to the large amount of small particle size diatomaceous earth required and the rapid increase in pressure drop resulting from the large amount of diatomaceous earth.
Diatomaceous earth filters are operated by precoating a support with diatomaceous earth filter aid prior to service, and then additional diatomaceous earth body feed is added to the stream to be filtered. The body feed continuously renews the filter surface and keeps the filter cake permeable. Thus, sticky solids are removed without either plugging the filter surface or plugging the filter cake. Diatomaceous earth capable of filtering fine particles of iron hydroxide must be of relatively small particle size. Diatomaceous earth must also be used in relatively large quantities to prevent the iron hydroxide from plugging the filter cake. Mechanical removal of iron hydroxides requires that filter cakes have small pores to capture the iron hydroxide. Consequently, one has to use fine grades of diatomaceous earth, which causes high operating pressures, low filtration rates, and requires large capital investments in high surface area filters.
Brines used in well completions and work-overs tend to accumulate iron hydroxides due to exposure to iron containing tubulars. Such brines are used as many times as possible because weighing components, fluid loss additives and other chemical additives cost a considerable amount of money. Generally, the brines are filtered through diatomaceous earth filters to remove suspended solids. Iron hydroxides will tend to accumulate in the brines because of the difficulty of removing iron hydroxides. Brines containing more than about 10 mg/l of suspended solids, including iron hydroxides, are not suitable for use in completion and work-over. It would be desirable to have a filter medium which effectively and economically removes iron hydroxides from brines and other water mediums.
It is therefore an object of the present invention to provide a method that can reduce iron hydroxides (in aqueous mediums) to levels of 10 mg/l and less with good filtering rates.